Question

I was hoping that I could setup a cmdlet so that it would use an environment variable for a parameter value if it exists, or otherwise prompt.

function Test-Mandatory
{
  [CmdletBinding()]
  param(
    [Parameter(Mandatory = { [string]::IsNullOrEmpty($Env:TEST_PARAM) })]
    [string]
    $foo = $Env:TEST_PARAM
  )

  Write-Host $foo
}

Unfortunately, it seems that regardless of whether or not I have a $Env:TEST_PARAM set, the cmdlet always prompts for $foo.

I could rework the validation to use [ValidateScript({ #snip #}), but then I wouldn’t get Powershell prompting for the required value anymore, should $Env:TEST_PARAM not exist. I would simply get a validation error.

So 2 questions here

• Why can I even assign a scriptblock to Mandatory if it doesn’t appear to be honored?
• Is there a simple way to get the default PS prompting behavior given the criteria I specified?

Answer 1

FYI, here’s what Microsoft had to say about it:

“It’s not really a bug. Script blocks are valid attribute arguments –
e.g. ValidateScriptBlock wouldn’t work very well otherwise.
Attribute arguments are always converted to the parameter type. In
the case of Mandatory – it takes a bool, and any time you convert a
ScriptBlock to bool, you’ll get the value $true. You never invoke a
script block to do a conversion.”